Efficient Singlet Oxygen Generation in Metal Nanoclusters for Two-Photon Photodynamic Therapy Applications.

The generation of singlet oxygen (1O2) has been established as the principal mechanism of photodynamic therapy (PDT). Various dyes, metal nanoparticles, and clusters have been shown to sensitize 1O2. However, metal nanoclusters are even more promising candidates as photosensitizers for this purpose. By understanding the optical properties that lead to efficient 1O2 generation, one can fully realize their potential as PDT photosensitizers. Three different metal nanoclusters, Au25, Ag32, and Au144, are investigated for their 1O2 generation efficiency. The Au144 showed a 1O2 generation rate that is 2 orders of magnitude higher than that for Au25 and Ag32, and several orders of magnitude higher than nanoparticles (>5 nm) due to Au144's high absorption cross section-to-volume ratio. The effectiveness of PDT in live cells with nanoclusters was demonstrated by two-photon excitation compared to one-photon excitation. The implication of these results points toward new efficient two-photon 1O2 sensitizers for photodynamic therapy.

Synthesis of Ladder-Type Thienoacenes and Their Electronic and Optical Properties.

A series of ladder-type thienoacenes based on benzo[1,2-b:4,5-b']dithiophene (BDT) have been synthesized and characterized. They were shown to be p-type semiconductors with wide band gaps and able to support multiple stable cationic states. As the conjugation lengthens, these oligomers become more emissive, showing high quantum yields. They were shown to be good two-photon absorbers, exhibiting high two-photon absorption coefficients.

Linear and Nonlinear Optical Properties of Monolayer-Protected Gold Nanocluster Films.

Gold nanoclusters have been extensively studied in solution for their unique optical properties. However, many applications of nanoclusters involve the use of the material in the solid state such as films. Au25(SR)18 in polymeric hosts was used as the model for studying the optical properties of nanocluster films. Different film-processing conditions as well as types of polymers were explored to produce a good-quality film that is suitable for optical measurements. The best optical film was made using Au25(C6S)18 and polystyrene. The formation of nanocluster films drastically reduces the intercluster distances to a few nanometers, which were estimated and characterized by optical absorption. The steady-state absorption and emission properties of the nanocluster film maintained their molecular characteristics. The emissions from the nanocluster films are found to be strongly enhanced at 730 nm with a smaller enhancement at 820 nm when the intercluster distance is below 8 nm. The emission enhancement can be attributed to the energy transfer between clusters due to the small intercluster distance. Two-photon Z scan revealed that the two-photon absorption cross sections are in the order of 10(6) GM, which is an order of magnitude higher than it is in solution. The two-photon absorption enhancement is correlated with strong dipole coupling. These results show that metal nanoclusters can be made into optical quality films, which increase the interaction between clusters and enhances their linear and nonlinear optical responses.

An ultrafast look at Au nanoclusters.

In the past 20 years, researchers studying nanomaterials have uncovered many new and interesting properties not found in bulk materials. Extensive research has focused on metal nanoparticles (>3 nm) because of their potential applications, such as in molecular electronics, image markers, and catalysts. In particular, the discovery of metal nanoclusters (<3 nm) has greatly expanded the horizon of nanomaterial research. These nanosystems exhibit molecular-like characteristics as their size approaches the Fermi-wavelength of an electron. The relationships between size and physical properties for nanomaterials are intriguing, because for metal nanosystems in this size regime both size and shape determine electronic properties. Remarkably, changes in the optical properties of nanomaterials have provided tremendous insight into the electronic structure of nanoclusters. The success of synthesizing monolayer protected clusters (MPCs) in the condensed phase has allowed scientists to probe the metal core directly. Au MPCs have become the "gold" standard in nanocluster science, thanks to the rigorous structural characterization already accomplished. The use of ultrafast laser spectroscopy on MPCs in solution provides the benefit of directly studying the chemical dynamics of metal nanoclusters (core), and their nonlinear optical properties. In this Account, we investigate the optical properties of MPCs in the visible region using ultrafast spectroscopy. Based on fluorescence up-conversion spectroscopy, we propose an emission mechanism for these nanoclusters. These clusters behave differently from nanoparticles in terms of emission lifetimes as well as two-photon cross sections. Through further investigation of the transient (excited state) absorption, we have found many unique phenomena of nanoclusters, such as quantum confinement effects and vibrational breathing modes. In summary, based on the differences in the optical properties, the distinction between nanoclusters and nanoparticles appears at a size near 2.2 nm. This is consistent with simulations from a free-electron model proposed for MPCs. The use of ultrafast techniques on these nanoclusters can answer many of the fundamental questions about the nature of these exciting nanomaterials and their applications.

Bright two-photon emission and ultra-fast relaxation dynamics in a DNA-templated nanocluster investigated by ultra-fast spectroscopy.

Metal nanoclusters have interesting steady state fluorescence emission, two-photon excited emission and ultrafast dynamics. A new subclass of fluorescent silver nanoclusters (Ag NCs) are NanoCluster Beacons. NanoCluster Beacons consist of a weakly emissive Ag NC templated on a single stranded DNA ("Ag NC on ssDNA") that becomes highly fluorescent when a DNA enhancer sequence is brought in proximity to the Ag NC by DNA base pairing ("Ag NC on dsDNA"). Steady state fluorescence was observed at 540 nm for both Ag NC on ssDNA and dsDNA; emission at 650 nm is observed for Ag NC on dsDNA. The emission at 550 nm is eight times weaker than that at 650 nm. Fluorescence up-conversion was used to study the dynamics of the emission. Bi-exponential fluorescence decay was recorded at 550 nm with lifetimes of 1 ps and 17 ps. The emission at 650 nm was not observed at the time scale investigated but has been reported to have a lifetime of 3.48 ns. Two-photon excited fluorescence was detected for Ag NC on dsDNA at 630 nm when excited at 800 nm. The two-photon absorption cross-section was calculated to be ∼3000 GM. Femtosecond transient absorption experiments were performed to investigate the excited state dynamics of DNA-Ag NC. An excited state unique to Ag NC on dsDNA was identified at ∼580 nm as an excited state bleach that related directly to the emission at 650 nm based on the excitation spectrum. Based on the optical results, a simple four level system is used to describe the emission mechanism for Ag NC on dsDNA.